Our work on developing sensitive double label protocols to determine precursor product relationships between proteins in different brain subcellular fractions will be continued. Rats, 18 days old, will be injected with radioactive amino acids and synthesis and transfer of myelin specific proteins (basic protein, proteolipid protein, and high molecular weight proteins) from denser to lighter myelin subfractions will be studied. We will attempt to confirm our preliminary data indicating that addition or proteolipid protein and certain lipids is the final committed step in formation of mature myelin. Possible perturbations in this assembly process will be studied in animals that are made hypothyroid by injection of methimazole, or in animals in which experimental allergic encephalomyelitis has been induced. The double label protocol which we have developed will also be used to study the transport and assembly of nerve ending proteins in developing rat brain. Another double label isotope methodology will be used to study the long term turnover of myelin lipids. Particular attention will be paid to the reutilization of various moieties of phospholipid molecules; i.e., why does the apparent half-life of lecithin vary depending on whether it is determined with radioactive glycerol, choline, or palmitate.